My theory is that the "thrill rides" (coasters, etc.) are all likely destined to be replaced (or at least substantially modified) someday because of the way the bar keeps getting raised on the scariness factor. I was at Busch Gardens last Saturday and we ate lunch next to the new Sheikra coaster, which features a 90 degree free fall -- I mean straight down and then immediately into an inverted loop. Just watching it gave me the creeps. I looked over across the way at the old Scorpion coaster, which seemed so deliciously sinister when I was a kid, but now seems almost quaint in comparison.
I think certain rides are icons -- Small World, Haunted Mansion, Pirates of the Carribbean, and they'll likely be there in substantially the same form for as long as there is a Magic Kingdom. To say "Small World" is boring misses the point. Little kids don't much care that Small World is ancient and corny compared to some other attractions. They're never going to be too jaded to be fascinated by a boat ride among hundreds of little moving colorful mannequins and a snappy little tune that you can't get out of your head for months. It's kind of like merry go rounds. How passe are they, but they're still a mainstay of every carnival and theme park. Somewhere around the age of 7 or 8 most children "grow out of" them. Before then, however, they'll never be able to pass one without begging for a ride.